The present invention is based on a fuel injection apparatus as generally defined hereinafter. A known fuel injection apparatus of this kind (German Offenlegungsschrift 30 11 376) is intended exclusively for an application in which separate injection nozzles in a Diesel engine for main fuel and an igniting fuel are supplied with a main fuel which does not readily ignite and is pumped by a high-pressure injection pump, on the one hand, and an igniting fuel which is pumped by a separate pump via a hydraulic auxiliary pump, on the other. In this known apparatus, a pilot injection piston in the auxiliary pump is driven at the supply pressure of the high-pressure injection pump by a piston of larger diameter and positively displaces a pilot injection quantity from a work chamber disposed before it to the separate nozzle for the pilot injection. In order to be able to adjust the stroke path of the pilot injection piston positively displacing the pilot injection quantity, and hence to be able to adjust the pilot injection quantity itself, during engine operation, the pilot injection piston has adjusting means which limit its stroke. In this known apparatus, however, the timing of the pilot injection and the main injection with respect to one another is structurally invariable because of a stop in the preceding work chamber, which is effective in the supply direction and fixes the end of the pilot injection piston stroke, and also because of the displacement volume of the larger piston. It would therefore be problematic or even impossible to attain a correct phase relationship between the pilot and the main injection with an injection apparatus of this kind, for the further reason that dependencies on the rpm and load resulting from the dynamic influence of the lines and throttle conduits cannot be precluded. Furthermore, it is impossible to vary the instant of injection for the pilot injection in an intended manner and in terms of its timing with respect to the main injection while at the same time retaining the means of determining the pilot injection quantity.
It is well known that undesirable operating noise in Diesel engines can be attributed to the very rapid liberation of energy at the onset of combustion, and so attempts have long been made to initiate combustion by means of a pilot injection quantity which is limitable and also positionable at the desirable time with respect to the main injection, and thereby to limit the speed of combustion. The solution which presents itself in this connection, that is, the disposition of two complete, separate injection systems functioning in parallel, is complicated and not recommended, because not only are two pumps, two lines and two nozzles required, but also the means necessary for synchronizing the two systems.
It is also known to attain pilot injection effects by the suitable dimensioning of a standard injection system; in this case, a predetermined relationship in terms of size and function must be maintained for the pilot stroke, line diameters, nozzle ports and nozzle springs. This provision, however, leads to a disadvantageous dependency on load and rpm and on the varying dynamic influences during engine operation.
It is also known to provide additional control devices and an intermediate reservoir in injection pumps, by which it is possible to reduce the supply speed by throttling down to the vicinity of zero. Since an initial step or gradation can build up in this case in the pressure wave traveling in the nozzle, it is possible, at certain rpm and load levels, to attain a sort of pilot injection.
Even if the pilot injection is performed in terms of its metering and timing position by two systems having two injection pumps the camshafts of which are coupled together, difficulties arise in attaining the correct phase relationship between the pilot and the main injection, because of the dependency on rpm and load resulting from the dynamic influence of the two lines.
To attain a pilot injection and main injection, another apparatus is also known (German Patent 1 252 001), in which a separate, small piston for the pilot injection is disposed axially parallel to but offset from a loading piston for the main injection inside a fuel injection valve. A separate supply of low pressure can thereby be dispensed with, and the pilot injection quantity is derived from the fuel supply for the main injection quantity--but this does not preclude a disadvantageous effect on the standing pressure in the pressure line and hence inaccuracy in terms of the quantity control.
Finally, for controlling the pilot injection quantity in internal combustion engines, it is also known (German Offenlegungsschrift 28 34 633) to provide a one-piece control slide which is displaceable counter to the force of a spring and which furnishes the various connections desired for pilot and main injection with a pronounced intermediate relief into a reservoir via control edges. Here, again, the pilot injection is diverted from the supply quantity of the injection pump that also provides the main injection quantity, so that the accuracy of quantity control for the main injection quantity is impaired.